There are many types of work machines which have a front boom as a component, as exemplified by hydraulic shovels.
The boom is often curved in the longitudinal direction and configured with a top plate forming an upper surface, a pair of side plates forming side surfaces, a bottom plate forming a lower surface, and a baffle. The baffle is a reinforcing wall provided to partition an internal space having the top plate, side plates and bottom plate as the peripheral walls and is welded to the inner wall surfaces of the top plate, the side plates and the bottom plate.
Japanese Patent Application Laid-open No. 2002-348903 discloses a boom structure capable of reducing torsional stress. This boom structure simulates a reference line that connects one end portion positioned on the boom base end side in a first bracket for fitting a boom cylinder attached to the bottom surface of the boom, and the center of curvature of a middle curved portion of the boom, wherein a lower reinforcing wall that is fixed to the bottom surface at one end portion is positioned in such a manner that a flange portion of the lower reinforcing wall approaching a top surface is inclined away from the reference line at the boom base end side. In this manner, the torsional stress occurring at a tip end portion of the reinforcing wall is reduced. The disclosed boom structure serves to reduce the torsional stress that occurs in the reinforcing wall and is not designed to prevent the stress from concentrating on the rear upper portions of the side plates.
When longitudinal bending, lateral bending, and twisting are made multiple times in the entire boom, high stress occurs in the boom. This stress tends to concentrate on the part where the baffle in the internal space of the boom and each of the plates (the top plate, the side plates, and the bottom plate) are welded together. The stress tends to concentrate especially on the top plate side of the welded parts between the rear side baffle and side plates, i.e., the rear upper portions of the side plates.
Such concentration of the stress can be prevented and handled by improving the strength of the reinforcement structure by increasing the plate thickness of the side plates and/or the baffle, but it unfavorably leads to significant increases in weight and cost.
The present invention was contrived in view of the foregoing problems, and an object thereof is to provide a reinforcement structure for a boom of a work machine, capable of preventing the stress from concentrating on the rear upper portions of the side plates while preventing the increases in weight and cost.